A process for monitoring the movement of closure devices which may be adjusted by motors, for example for use in conjunction with power windows, has become known in German patents DE 30 34 118 C2 and DE 31 36 746 C2. That reference discloses Hall sensors which are allocated to the power window motor and provide one signal per motor period, one motor period being one complete revolution of the motor. The signals are intended to determine the distance which has been travelled by the adjustable closure device, which shall hereafter be referred to as a window, or its current position, and they also determine the time taken by any given current motor period. This monitoring is generally active within a central portion of the window travel. The current motor period time value is compared with a threshold value derived from a previous motor period time value which is subject to continuous updating. If the current motor period time value is greater than the threshold value, the power window motor is switched off or reversed because it is probable that the movement of the window is being influenced or blocked by an obstacle.
This type of monitoring can generally be considered satisfactory. However, when the power window is switched on, monitoring only begins after one full motor period, because before then no prior motor period time value is available. This means, that after short operations of the operating elements, in other words, with activation of the power window motor for a time which is shorter than the time necessary for a full motor period, the window can be moved without check against an object which may be blocking the window, thus causing possible danger or injury to the same. In addition, as the motor period varies quite considerably during the start-up phase of the motor, a stable state, or even running of the motor, is only reached after about three motor periods. Hence, satisfactory protection in the case of excessive force being produced against the motor can only be assured after this time.